Semiconductor modulator



Aug. 7, 1962 E. G. I INDER SEMICONDUCTOR MODULATOR Filed April 30, 1957 2 Sheets-Sheet 1 ERNEST E. UNDER BY/Yzfmf 3,048,797 SEMICONDUCTR MODULATOR Ernest G. Linder, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 30, 1957, Ser. No. 656,003 11 Claims. (Cl. 332-31) This invention relates to improved microwave apparatus employing semiconducting elements. More particularly, the invention relates to novel apparatus for producing magnetron-type microwave oscillations of charge-carriers in a semiconductor element subjected to a magnetic field.

Heretofore, the magnetron has been a vacuum tube essentially of the diode type wherein the electrons flowing from the cathode to the plate under the influence of an electric eld existing therebetween Vare made to follow a cyclic or spiral path by means of a magnetic field established perpendicular to the electric field. The strength of the magnetic field is increased beyond the point whereat the electrons are caused to curve sufficiently to miss the plate and follow a cyclic orbit. Oscillations at an ultra high frequency are produced by virtue of the currents induced electrostatically by the moving electrons. As used herein, the term magnetron-type motion is intended to mean the flow pattern or movement of electrons or charge carriers when subjected to mutually perpendicular magnetic and electric lfields. The frequency of oscillation is roughly determined by the time it takes the electrons or charge carriers to perform a complete cycle of motion. The ultra high frequency energy generated is transferred to a load by means of an external circuit between the cathode and plate of the magnetron.

A magnetron tube uses an evacuated envelope to protect the thermionic cathode from exposure to air, and to permit unimpeded electron flow in the tube. A magnetron tube also may use a complex multiple cavity resonator anode having sufficiently high Q to sustain oscillations in the tube. A substantial heater current is required atleast initially to heat the thermionic cathode, and a very strong magnetic field must be supplied by a magnet or solenoid. In addition, the only practical methods for modulating a vacuum-tube magnetron are by varying the anode voltage, or by modulating a second electron beam projected through the resonant structure.

An object of this invention is to provide improved methods of and means for generating electromagnetic waves in the microwave frequency range.

Another object of this invention is to provide improved methods of and means for generating and modulating electromagnetic waves in the microwave frequency range.

Another object of this invention is to provide improved methods of and means for utilizing a semiconductor element for generating microwave energy.

A further object of this invention is to provide an improved microwave generator which requires no evacuated envelope in its structure.

Another object of this invention is to provide an improved semiconductor microwave generator utilizing charge-carriers subjected to a magnetic field.

Another object of this invention is to provide a microwave generator requiring no thermionic cathode.

Another object of this invention is to provide a microwave generator which may be modulated by a number of techniques in addition to varying the anode voltage.

These and other objects and advantages may be accomplished in accordance with the present invention by apparatus wherein magnetron-type motion is imparted to charge-carriers in a semiconductor element. When an electrostatic iield is impressed across the semiconductor element, charge-carriers, which may be created therein by means and techniques wellllrnown in the semiconductor ited States Patent art, flow between electrodes which establish an electrostatic field. By immersing the semiconductor element in a magnetic field, perpendicular to the electrostatic field, the charge carriers will flow in a magnetron-type motion, and microwave oscillations may lbe sustained by la resonant circuit in cooperation with the semiconducting element. By varying the electrostatic field or by varying theV number of charge carriers created, the generated microwave oscillations may be modulated.

The invention is. described in more detail in connection with the Vaccompanying drawings, in which:

FIG. l is a schematic top section View of a split-anode type semiconductor magnetron;

FIG. 2 is a schematic top section view of an opticallyexcited semiconductor magnetron; y

FIG. 3 is a schematic top section view of a semiconductor magnetron excited by radioactive bombardment;

FIG. 4 is a schematic top section lview of a semiconductor magnetron using a multiple cavity resonator and waveguide output;

FIG. 5 is a schematic top section view of a semiconductor magnetron `contained within an output waveguide;

FIG. 6A is a `schematic side section 'View of a semiconductor magnetron utilizing a p-n junction device; and

FIG. 6B is ak schematic side view in section of another embodiment of the semiconductor magnetron of FIG.` 6A using a pair of p-n junction devices.

Similar reference characters are applied to similar elements throughout the drawings.

Referring now to FIGURE 1, a hollow cylinder 1 of p-type semiconductor ymaterial is oriented withits axis perpendicular to the plane of the figure. The semiconductor may be germanium, silicon, gallium arsenide, or indium phosphide, for example. intrinsically pure mag terials may be employed or the materials may be doped with other elements to establish a particular type of conductivity. N-type conductivity, that is, current conduction which is due to negatively-charged carriers or electrons, may be established in germanium or silicon by irnpurities having an excess of electrons in their valence band such as phosphorus, arsenic, and antimony, for example. PJtype conductivity, that is, current conduction, which is due to positively-charged carriers or holes, may tbe established in germanium or silicon by impurities having a deficiency of electrons in their valence band such as gallium and indium. Gallium arsenide and indium phosphide may be made to have p-type conductivity by such impurities as cadmium, zinc or mercury; n-type conductivity is established in these semiconductors by sulfur, selenium, or tellurium. Preferably, the semiconductor element is single crystalline. Polycrystalline material may be employed although less eiciently since crystal boundaries will interfere `somewhat with charge carrier motion. For optimum efficiency, the semiconductor material is preferably selected to have its carrier mean-free-time no less than a full period of oscillation at the operating frequency.

The material is formed into a cylinder by means of an ultrasonic wibratory cutting instrument, for example, although other techniquesmay be employed such as etching. Illustratively, the cylinder may be about 1 mm. high and have a diameter of about 0.5 mm. The wall thickness ofthe cylinder may be about 0.2 mm. A metal tubular member 5 is lfitted to contact the inside surface of the hole 7 of the semiconductor cylinder 1 to constitute a cathode electrode and to form a metal-semiconductor contact. rThe contact, in the instant embodiment, may be either ohmic or rectifying. In either instance it serves the dual purpose of cooperating with the outer metallic plates 3` and 4 to establish an electric field through the semiconductor body 1 and to inject charge carriers into the semiconductor. The tubular member 5 may be in 3 'pressure contact-with the inside of the semiconductor cylinder 1, or it may be constituted by a plated metal layer thereon, or it may be soldered thereto. Metallic plates 3 and 4 in the form of cylindrical segments contact y'the .outside surface of the cylinde'rto constitute anode electrodes of the split-anode type. A two-wire transmission line, formed by the conductors 11 and terminated by 'the terminating 4conductor or tuning element 13, Yis. con- 'nected to the` anode electrodes 3 and 4 thus forming a "resonant section of line. The tuning element 13 is moved along between the conductors 11 to adjust the' inductance 'of the loop 11, 13,11 and thus to vary the frequency of oscillation. The power supply/15 is connected to the 'tulbular member 5 or cathode and to the midpoint, or voltage node, of the tuning element 13. Thus the transmission line conductors 11 and the split anodes 3 and 4 are maintained at a suitable anode voltage such that the charge carriers will make one or more cycles of motion before reaching the anode. The voltage required will de- -pend upon the desiredfrequency to be generated, the dimensions of the semiconductor body, the eiective mass of the charge carriers (which is determined by the kind of material), and by the mod e of operation (i.e., splitanode mode, low-iield mode, etc.). The potential difference between the cathode tubular member and the anode electrodes 3 and 4 forms an electrostatic Alield having a radially directed potentialgradient in the semiconductor. In this embodiment, the tubular member 5 acts as a source of electrons (since the semiconductor element 1 is of ptype conductivity) due to one or more of the effects of carrier injection, eld breakdown, and impact ionization caused by the radial potential gradient in the semiconductor cylinder. As is known in the art, the polarity of the applied yvoltagel and the type of semiconductor determine'whether either electrons or holes are generated. Hence, in the present embodiment, wherein p-type material is employed, the tubular electrode 5 is made negartive with respect to the semi-cylindrical electrodes 3 and 4.

A magnetic field is estalblished in the semiconductor element 1 by =a solenoid coil 8, for example. The eld established is parallel to the axis of the cylindrical element 1, and the lines of force of the magnetic eld are perpendicular to the radial potential gradient of the electrostatic iield. Hence, the flow of current from the tubular electrodeS yto the electrodes 3 and 4 is thus modified by the presence of the magnetic lfield, and the trajectories of the electrons assume a magnetron-type path. The strength yof the magnetic field must be sutlicient to give cyclic motion to the electrons at the frequency desired to be generated. The resonant circuit 11 and 13, acting together with the current in the semiconductor, sustains microwave oscillations. The portions 17 of the transmission line formed by the conductors 11 form an output circuit coupled to themicrowave load 18.

Referring again to FIGURE l, in a modification of the split-anode embodiment of the Vinvention iirst described, a heater v9, located internally the semiconductor element 1, heats the -tubular member 5 :and the portion of the semiconductor adjacent thereto resulting in thermionic excitation of electrons or holes in the semiconductor. Thus in this embodiment the tubular mem-ber 5 may but does nothave -to function as a means for injecting charge carriers into the semiconductor; its primary function is to cooperate with the metallic plates 3 and 4 in establishing an electric field through the semiconductor. Leads 21 apply a suitable voltage for heater operation to the heater 9. The magnetic and electrostatic fields are established as beforepandcause the emitted electrons to move in magnetron-type paths as described, supra. The operationV ofthe resonant circuit, the output coupling lines and the electrodes 3 andS are the same as in the iirst embodiment.

Referring to FIGURE 2, another embodiment with a cylindrical semiconductor element 1 is now shown partially in section with its axis lying in the plane of the figure.

4 Light energy from a source represented by the light bulb 23 irradiates the semiconductor element 1 and also falls on a solid transparent rod25 which may be made of quartz, for example. The quartz rod 25 is coated with a transparent conducting layer 27, the outer surface of this -layer contacting the inner surface of the semiconductor 1. A concentrating lens 29 may ybe used to increase the light intensity incident on quartz rod 25. Light falling on the semiconductor 1 may partially penetrate the semiconductor, which has some transparency to light. The quartz rod 25 conducts light along its length by means of multiple internal reiiection. Thus light energy irradiates the inner surface of the semiconductor cylinder 1 adjacent to the quartz rod 25. An electrostatic field is established with a radial potential gradient in the semiconductor element 1 by means of electrodes V3 and 4 and the internal electrode 27 which is constituted by the transparent conducting layer located internally With the semiconductor element 1. The electrodes 3, 4 and 27 are connected to the power supply 1'5 so as to maintain an anode voltage sufficient to cause the charge .carriers to make one or more cycles of motion before reaching the anode. VIt should be appreciated that if the semiconductor element 1 is of p-type conductivity and the central electrode 27 is made negative with respect tothe semi-cylindrical electrodes 3 and 4, then negative charge carriers or electrons will flow from the cathode electrode 27 to the anode electrodes 3 and 4. Onthe other hand, by making the central electrode 27 positive with respect to semicylindrical electrodes, 3 and 4, then positive charge carriers or holes will ow from the anode electrode 27 to the cathode electrodes 3 and y4. If the semiconductor element 1 is of n-type conductivity and .fthe central electrode 27 is negative with respect to the semi-cylindrical elements 3 and 4, then negative charge carriers or electrons will flow from the cathode electrode 27 to the anode electrodes 3 and 4. Conversely, positive charge carriers or holes will flow from the central electrode 27 to the semi-cylindrical electrodes 3 and 4 if the central electrode 27 is positive with respect to the semicylindrical electrodes 3 and 4. A magnetic eld whose lines of force are perpendicular to the direction of the electrostatic rfield is established by the solenoid coil 8, for example, which is coaxially disposed around the semiconductor cylinder Y f Radiation from the light source 23 causes the release of electrons or holes in the semiconductor by optical excitation of electrons into the conductive band. A current of electrons or holes thus flows in the semiconductor to the electrodes 3 and 4 in magnetron-type paths as described previously. The remainder of the operation of the resonance circuit, the output coupling lines and the micro- Wave load are the same as have been described previously.

The semiconductor magnetron may have its operation considerably enhanced by cooling it. Lowering the temperature reduces latticescattering in the semiconductor crystal and thus produces a ylonger mean free path which permits the electrons or holes `toachieve a larger number of cycles of motion without disturbance due to scattering. Hence the semiconductor magnetron may be contained in a cooling device represented bythe dotted lines 10. The cooling device may be a Dewar flask, for example, in which the semiconductor device 1 is contained and subv stantially surrounded by liquid nitrogen or helium, for

example. i Y

Another embodiment is shown in FIGURE 3 wherein a metallic or electrically conductive tube 5 is passed through the semiconducting cylinder 1 and is filled with a radioactive material 31. The end caps 33 Iare provided on the tube 5 to retain the radioactive material. The end surfaces of the semiconductor cylinder 1 may also be coated with radioactive material 35. Electrons or holes are released within the semiconducting material by radioactive bombardment. The electrostatic iield is established as described heretofore. The magnetic field H is established by means of the magnetic pole pieces 34 and 47 so that the eld is substantially perpendicular to the electric tield. The electrons or holes ow in magnetron-type paths, and the resonant circuit and output coupling means'to a microwave load are the same as those described previously.

In another embodiment, shown in FIGURE 4, a semiconducting cylinder 1 is coaxially oriented within a multiple cavity type resonator 41. The semiconductor cylinder 1 contacts the inside portions 45 of the cavity resonator which lie between individual cavity slots 47. The contacting portions 45 -function as anode electrodes and are an integral part of the resonator 41. The cavity slots 47 sustain microwave oscillation. A microwave coupling means 49 provides a tapered transition to a waveguide portion 51 and is coupled to one of the cavity portions 47 Microwave energy is delivered to a microwave load 18 through the waveguide coupling means 49. An electrostatic lield is established with a radial potential gradient by means of the lsource of power 15 which is connected to the tube S and the resonator 41. A magnetic iield is established by means of a solenoid 8l coaxially disposed around the resonator-semiconductor assembly. Magnetron-type current flow occurs here as described before.

Referring to FIGURE 5, Va semiconductor cylinder 1 is completely contained within a waveguide 53. Arcuate electrodes 3 and 4 contact the outside surfaces of the semiconductor cylinderl. A tubular member 5 contacts the inner surface of the semiconductor. The arcuate electrodes 3 and 4 and the tube 5 are connected to a power source 15l to provide a radial potential gradient in the semiconductor cylinder as has been described. A lead V55 passes through the waveguide wall and is electrically insulated therefrom by the insulator S6, connecting the tubular member 5 to the power source 15. The electrodes 3 and 4 are connected by leads 57 to appropriate portions of the waveguide wall which, in turn, is also connected by leads -to the power source 15. The portion of waveguide contacting the leads 57 'functions ras a resonant circuit. The magnetic iield is established by the solenoid 3 as described.' Microwave energy is propagated down the waveguide 53 to a microwave load 18.

Another embodiment is shown in FIGURE 6A wherein a semiconductor p-n rectifying ljunction device 61 is immersed in a magnetic field established `b-y means of the magnetic pole pieces 70 and 74. The semiconductor junction device 61 is contained in a waveguide section 63 which is tapered to a smaller portion 67 to provide impedance matching. The semiconductor junction device 61 is composed of a p-type region 64 comprising germanium doped with arsenic, yfor example, an n-type region 66 comprising germanium doped with indium, for example. 'I'hese regions are fused together or otherwise contact each other so that they form a p-n rectifying junction 69 at the interface between the p-type and n-type regions. The n-type region is in electrical contact with the ywaveguide 63 at the portion `67 thereof, and the p-type region is in electrical Contact to the electrically conductive contact member 71. The power supply 15 is connected to the waveguide 63 and the contact member 71, and impresses a potential difference 'across Ithe semiconductor junctionfdevice 61 by supplying voltage between the contact member 71 and the waveguide portion 67. The contact member 71 vis electrically insulated from the waveguide 63 by the gaps 73. 'Ihe waveguide characteristics of continuity of microwave propagation are maintained by capacitive microwave coupling between the contact element 71 and the waveguide 63 `across the gaps 73. An impedance matching adjustment is provided by the sliding conducting plug 75 which makes contact to the walls of the waveguide 63. A magnetic eld is established perpendicularly to the electric eld by means of the tapered magnetic pole pieces 70 and 74, for example, the magnetic lines of `force curving down and through the device 61. v In operation, carrier injection occurs at the p-n junction 69 in the semiconductor device 61. Iif the waveguide portion 67 is made negative with respect to the contact member 71, then negative charge-carriers (electrons) migrate from the junction `69 through the p-type region 64 toward the contact member 71, due to the electric field in the material. If the polarity is reversed, then positive charge-carriers (holes) -will flow from the junction through the n-type region 66 toward the waveguide portion 67. The path of the electron or hole llow is modified by the presence of the perpendicular magnetic lield, and magnetron-type trajectories result. One such trajectory of electrons is illustrated at 77. These electrons in cyclic trajectories sustainmicrowave generation, similar to that in magnetron vacuum tubes. The microwaves are propagated in the waveguide 63 to the-microwave load 18.

In FIGURE 6B a double junction 72 is shown which may be incorporated in the ywaveguide apparatus of FIGURE 6 in lieu of the single junction device 61. The device 72 actually comprises a pair of p-njunction devices so arranged that their respective n-type regions 73 and 75 contact a common electrical conductor 74. The p-type regions 76 and 78 are contacted to the contact member 67 andwaveguide portion 71, respectively, as in the embodiment of FIGURE 6A. The power supply 15 is connected to the central electrode 74 and to the contact member 67 andthe waveguide portion 71 as shown. If

vthe semiconductor magnetron device.

the central electrode 74 ismade negative with respect to the contact member y67 and the waveguide portion 71, then negative charge-carriers (electrons) will migrate from the junctions and 82 through the p-type regions 76 and 78, respectively toward the contact member 67 and the waveguide portion 71, respectively. Likewise, positive charge-carriers (holes) will migrate through the n-type regions 73 and 75 toward the central electrode 74. Oscillations of much higher current are thus obtainable with this arrangement.

It should be appreciated that much higher frequencies may be generated by the `semiconductor magnetron device described 4than the frequencies generated by a vacuumtype magnetron because theeffective mass of chargecarriers (electrons or holes) in a semiconductor is less than the effective mass of electrons in free space or vacuum. Frequencies obtainable with vacuum-type magnetrons are also obtainable with the semiconductor -magnetron device described herein but at electric and magnetic iield strengths substantially lower than the iield strengths required by the vacuum-type magnetrons.

The semiconductor magnetrons shown and described herein may have their outputs modulated by varying the anode voltage. Modulation may also be achieved by varying the number of charge-carriers created in the semico-nductor body las, `for example, by varying the intensity of the light'source 23 in the embodiment of FIGURE 2; the light beam might also be chopped-as by the chopper 12 or deflected at the desired modulation rate. A third way of modulating the device is by varying the temperature as, for example, in the embodiment of FIGURE l wherein charge-carriers are thermally produced. Varying the temperature of the heater 9 (as by varyingV the heater current supplied thereto) modulates the output of Changing' the temperature ofthesemiconductor magnetron changes the number of electrons and lcharge-carriers to be raised lf rorn the conduction to the valence band, for example. Hence the device reacts to produce a larger or smaller number of charge-carriers in response to temperature changes.

By varying the anode voltage in the embodiment of FIGURES 6A and 6B, the bias across the junction (or junctions) is also changed, the result of which is to determine the number of minority charge-carriers injected across the junction (or junctions). Thus modulation of these junction-type magnetron devices is achieved by 'a two-fold effect: (l) a varying anode voltage and (2) a varyingv bias `across the junction.

ago-rave?.V

There thus has been shown and described a novel and improved magnetron device which does not require a vacuum tube or thelike and whichdoes not require a thermionic cathode and the concomitant substantial heater current. The semiconductor magnetron described herein is capable of operating at higher frequencies while requiring relatively lower electric and magnetic iield strengths, and several methods of modulating the output of the device have been described.

What is claimed is:

1. Apparatus comprising in combination means for providing a magnetic leld, a semiconductor body immersed in said magnetic ield, a source of charge carriers in said semiconductor body, and means lfor causing sard charge carriers to travel along a spiral path including anode and cathode electrodes for said semiconductor body for producing an electrostatic eld therein which is substantially radial `around a central axis thereof and substantially perpendicular to said magnetic field, microwave transmission means coupled to said body, and a microwave load coupled to said transmission means.

2. Apparatus comprising in combination means for trodes for establishing an electrostatic field in said semiconductor body perpendicular to said magnetic field` whereby'spiralfcurrent ow is sustained in said semi- 4. A semiconductor device comprising in combination 1' a semiconductor body, a tunable microwave resonant structure coupled to said body, means for generating free charge carriers in said body, means -for imparting spiral lmotion to said charge carriers, means for modulating the creation of said charge carriers, microvrave'transmission means coupled to said body, and a microwave Vload coupled to said transmission means.

5. Apparatus comprising in combination a semiconductor body, means for `generating `free charge carriers in said body, an anode electrode for said semiconductor j in said semiconductor body, va tunable microwave resonant Structure coupled'to said body, means for establishing a magnetic Viield insaid semiconductor body perpendicular to said electrostaticeld, microwave transmission means coupled to said body, and a microwave load cou- Apled to `said transmission means.V

6. 'Apparatus' comprising in combination a semicon- 'ductor ybody immersed in a magnetic field, means ffor gen- `erating free charge carriers in said body, ya resonant cavity'anode electrode surrounding said semiconductor body trode for said semiconductor body cooperating with said anode electrode to establish an electrostatic eld in said body perpendicular to said magnetic field, a tunable microwave resonant structure coupled to said body, and a microwave waveguide section coupled to said cavity anode electrode by means of `one of said slots.

7. Apparatus comprising in combination a hollow cylindrical semiconductor body immersed in a magnetic iield whose lines of force are parallel to the longitudinal axis 'of said body, means `for generating free charge carriers in said Ibody, `a cylindrical cavity anode electrode surrounding said body forming a plurality of slots therewith, a cathode electrode disposed within said cylindrical semiconductor body and cooperating with said anode electrode to establish a radially directed electrostatic iield therein,

a tunable microwave resonant structure coupled to said body, Yand a microwave Vwaveguide section coupled to said cavity anode electrode by means of one of said slots.

8. A semiconductor device comprising in combination: a semiconductor body, means for producing charge carriers in said body, means for establishing yan electrostatic iield in said body including a separate source of radiant energy and means for directing said radiant energy from -said source upon said semiconductor body, means for simultaneously establishing a magnetic eld in said body perpendicular to said electrostatic eld, a microwave resonant structure operatively associated with said body, microwave transmission means coupled to said body, and a microwave load coupled to said transmission means.

9. Asemiconductor device according to claim 8 comprising optical energy means yfor, producing free charge carriers in said body. l

l10. A semiconductor device according to claim 8 comprising radio-active means for producing free charge carriers in said body.

11. A semiconductor device Iaccording to claim 8 comprising thermal means for producing lfree charge carriers in said body.

References Cited in the le of this patent UNITED STATES PATENTS 1,856,865 Da rrah May 3, 1932 7.45 2,517,120 Linder Aug. l, 1950 2,553,490 Wallace May 15, 1951 2,691,138 Schwartz Oct. 5, 1954 2,691,736 *Haynes Oct. 12, 1954 2,695,930 Wallace Nov. 30, 1954 R0 2,743,322. Pierce et al. Apr. 24, 1956 "i 2,800,617 Pankove July 23, 1957 2,824,977 Pankove Feb. 25, 1958 2,911,601 Gunn et al. Nov. 3, 1959 2,944,167 Matare July 5, 1960 F155 FOREIGN PATENTS 1,129,061y France Sept. 3,1956

OTHER REFERENCES 60 ISllhe Physical Review, October 15, 1953, pgs. 215 to and -forming a plurality of slots therewith, a cathode elec- Transistorsg Theory and Application, by Coblenz et al., published by McGraw-Hill Book Co. Inc., New York, N.Y., pgs. 249-254, v 

